In the field of solar electricity generation, reflectors are commonly used which are parabolic in one plane only and extend linearly perpendicular to that plane such that they are symmetrical about that plane. With such two dimensionally parabolic reflecting surfaces, all radiation which is parallel to the axis of the parabola and is incident on the reflecting surface is focused at a focus line.
These prior art devices incorporating parabolic mirrors suffer from the disadvantage that the parabolic shape is very difficult to generate and manufacture. Despite being well defined mathematically and geometrically, a mirror with a parabolic shape is complex to create and beyond the capabilities of most workshops. Thus, so-called parabolic mirrors are often only approximations to parabolas, and are significantly less efficient than true parabolas. Allied to this problem is the high cost of complicated testing of the parabolic surfaces.
Furthermore, with the increasing dimensions of modern parabolic mirrors for astronomical applications (parabolic antennae having diameters up to 76.2 m (250 ft) are known), there is also the difficulty of maintaining the parabolic shape of the mirrors against the effect of gravitational forces. The above problems are equally true for circular, elliptical and hyperbolic reflecting surfaces.
One solution to the above problem of maintaining, for example, a parabolic shape against the effects of gravitational distortion is referred to as "active optics". A plurality of movable supports for a parabolic mirror are adjusted under computer control after an image analysis. This active optics adjustment of a mirror is limited to only slight corrections to the optical characteristics and maintenance of the parabolic shape. Active optics provides no solution to the problems of manufacture of a parabolic mirror.